One type of passive restraint for safeguarding automotive passengers from injury is the inflatable envelope or cushion which, in the event of a collision, is filled by an inert gas generated through the controlled combustion of a solid chemical.
Whereas relatively compact gas-generating units are used for restraints facing the driver in the case of an accident, larger cylindrical generators are used for the passenger-side which utilize means for diffusing the generated gas in advance of inflating the envelope.
The generator typically comprises a main chemical generant or propellant in, for example, pressed pellet form, packed in a cylindrical canister and surrounding a booster charge. An igniter responds to a signal from a collision-sensor and causes an ignition cord within the booster to fire the booster which, in turn, ignites the propellant. The gas generated by the propellant is filtered, cooled, and passed through ports in a surrounding gas diffuser into the restraint cushion, typically a fine-mesh nylon bag.
The volume, pressure and temperature of the gas at various stages during the very brief time interval between the sensing of a crash and the full deployment of the cushion, as well as rates of ignition, burning, and gas-generation may be tailored by means well-known within the art.
It is known that, in general, as the ambient temperature increases, the burn-rate of a solid propellant tends to increase. The effect of this increase in burn-rate is that the gases are produced at a much higher volumetric rate. Consequently, at the higher ambient temperatures, the system operates at higher pressures, and the deployment velocity of the cushion increases.
Additionally, as the propellant and filter become heated, the temperature, and hence the volume, of the delivered gas is increased. It is therefore desirable that the tailored operation of the gas generator proceed as uniformly as possible, whether under hot or cold ambient temperatures, i.e. with a minimal spread between resulting hot and cold operating conditions.
Although it is known, as for example through U.S. Pat. No. 4,191,392 to Barnett, to provide a vent at one end of the gas generator in order to prevent gas pressure build-up at that end, such vent merely redirects pressure, so that it still enters the cushion, but through the vent at the end of the generator rather than through the ports in the diffuser.
Such venting does not, however, contemplate the varying conditions of ambient temperature and gas flow rate which may obtain at the particular moment when firing takes place.